In many medical procedures, it is necessary to inject fluid into or withdraw fluid from a patient at relatively high-pressure. In addition, the amount of pressure needed to inject or withdraw fluid typically varies from procedure to procedure and from patient to patient. Therefore, the user must carefully monitor and control the syringe to ensure that the proper volume of fluid is dispensed or withdrawn at the appropriate rate. In such procedures, it is desirable to use hand-held syringes so that a trained medical staffperson can moderate and precisely control the amount of fluid injected into or withdrawn from the patient.
The need for syringes capable of injecting or withdrawing fluid at high pressure, wherein the volume of fluid injected or withdrawn can be carefully controlled, led to the development of hand-held "control" syringes. These control syringes are constructed much like traditional syringes, with a plunger slidably mating with the interior of a syringe barrel. Both the plunger and the syringe barrel in the control syringe are larger and made of stronger material than conventional syringes. These modifications allow the control syringes to withstand the higher pressures incurred in certain procedures.
Today, control syringes are used in a variety of medical procedures, especially angiographic procedures. For example, in one common use, a control syringe and a catheterization system are connected using a Luer connector. The control syringe is loaded with a dye that is injected into the coronary arteries of a patient. The dye usually must be injected under high pressure. An X-ray machine photographs the flow of dye through the cardiac system, thereby identifying any occlusions that are blocking blood flow in the coronary system. To accurately administer and calculate the flow of dye through the coronary system, both the pressure at which the dye is injected and the volume of dye injected must be controlled. Control syringes permit such high-pressure control of an injectate.
Control syringes are also used in other angiographic procedures, such as renal or carotid and angiographic procedures. The common use of control syringes in these procedures is the provision of a high-pressure, hand-held syringe capable of injecting or withdrawing specific volumes of fluid.
It is often the case in use of a control syringe that the user must exert maximum force on the plunger and syringe barrel to create the pressure needed to inject or withdraw fluid. Considering the amount of pressure that must be placed on the plunger and syringe barrel, it is not surprising that a common problem experienced when using control syringes is that users are unable to accurately control the volume of fluid expelled from or drawn into the syringe.
For example, presume the user must inject 2 cc of dye every 30 seconds for two minutes. The control syringe would be loaded with 8 cc of dye (2 cc.times.4 injections). The user may need to exert great force on the plunger to inject the fluid, while simultaneously controlling the plunger to inject only 2 cc at a time. In the past, users frequently applied pressure for too long and injected too much fluid. For example, the user might accidentally inject 4 cc of fluid instead of 2 cc, because the high pressure needed to move the plunger in the syringe barrel hindered the control of the plunger.
In order to control and limit the motion of the plunger within the syringe barrel, locking devices have been developed for angiographic control syringes. Typical locking devices are positioned on the plunger stem, and are movable along the vertical axis of the plunger stem. When the locking ring abuts the top of the syringe barrel, the plunger may be depressed no further within the syringe barrel. The locking device thus limits the amount of fluid that can be expelled from the syringe.
One such locking device is set forth in International Patent Application No. PCT/US89/0057, assigned to Merit Medical Systems Inc. ("Merit"). In Merit, a nut is slidably positioned on a plunger stem. A protruding member extends inwardly from the nut and engages a vertical slot on the plunger stem. The nut slides freely along the stem when the protruding member engages the vertical slot. A plurality of notches extend perpendicularly from the vertical slot. The protruding member on the nut may be aligned with a notch and, by rotating the nut, the protruding member engages the notch. When the protruding member engages one of the notches, the nut may not be vertically moved on the plunger stem. The nut limits how deep the plunger may be inserted into the syringe barrel, thereby limiting the amount of fluid that can be injected from the syringe control.
However, the above locking mechanism has deficiencies. In the Merit device, the nut has some rotational "play" when the protruding member is positioned in the vertical slot. The user may mistake this rotational play for the successful engagement of the protruding member with a notch. The nut does not preclude plunger movement if the nut is not locked into place. The danger of not properly inserting the protruding member within a notch is significant. For example, presume that the syringe barrel contains 10 cc of dye, but only 2 cc are to be injected at a time. The nut would be placed over a notch so that only 2 cc would be dispensed. However, if the protruding member does not fully engage the 2 cc notch, the protruding member will be aligned with the vertical slot. The user, expecting the nut to preclude the injection of more than 2 cc of fluid, will exert full pressure on the plunger stem. Instead of engaging the top of the syringe barrel to preclude downward motion of the plunger, the nut will slide in the vertical slot, resulting in injection of the full contents of the syringe. Further, if the protruding member only partially engages the notch, the nut may be dislocated from the notch by low force. This again may result in failure of the nut to preclude motion of the plunger in the syringe barrel.
A further problem with the Merit device is that the rotational movement required to lock and unlock the nut is a complex motion. The user of the control syringe is often using one hand to control the syringe barrel and the other hand to control the plunger. Also, each hand is usually pushing or holding part of the control syringe under great force. The need to apply pressure using both hands hinders rotational movement of either hand to control the nut.
Specifically, a user might hold the syringe barrel in their left hand. The user could then push or withdraw the plunger using their right hand. Both hands direct force onto the respective cylindrical objects (syringe and syringe barrel) under control. At least two fingers, typically the thumb and forefinger, must be relieved from holding the syringe or syringe barrel under force to control the locking nut. Tightly holding an object while trying to turn another object is awkward and difficult.
A different problem frequently encountered when using present control syringes is the presence of air bubbles within the injectate. The injection of an air bubble into a patient is undesirable and to be avoided. This problem led to the development of transparent angiographic syringes, which allow the user to view and screen the injectate for air bubbles. However, the users of control syringes sometimes do not see air bubbles in the injectate, especially small air bubbles. In present control syringes, the plunger head forces air bubbles downward toward the bottom of the syringe barrel. The frustoconical plunger head fully engages a seat at the bottom of the syringe barrel. Because the plunger head fully engages the seat of the syringe barrel, the entire contents of the control syringe are expelled. Therefore, any air bubbles in the syringe barrel are injected into the patient.
Accordingly, a need yet exists for an improved angiographic control syringe. Such a control syringe would have a locking mechanism that was easily adjustable and that locked securely and accurately into position. Further, such an improved control syringe would have means for capturing air bubbles in the injectate to limit the injection of the air bubbles into patients.